1. Field of the Invention
The present invention relates in General to an I/O (input/output) terminal for an electronic circuit device, such as an external lead or pin for an IC (integrated circuit) package, and more specifically to such a terminal lead or pin that is to be joined by gold (Au) solder to a thin-fim or thick-film metallized pad formed on a wiring base or package substrate.
2. Description of the Prior Art
A high density IC package, as disclosed in Japanese Patent Provisional Publication No. 2-101545, includes a multilayer substrate and a thin-film wiring section formed on the substrate. In some instances, the IC package further includes a sealing cap or cover made of Koval or the like and attached to the substrate. A high density IC package of a certain kind includes at the top or bottom surface of the substrate a plurality of conductive pads connected to the interior circuit and a plurality of I/O terminals such as external leads or pins made of Koval or the like and connected to the respective pads.
The multilayer substrate consists of a plurality of insulating layers made of alumina or the like ceramics, and various wiring patterns made of a high melting point metal and formed on the main surfaces of the insulating layers, respectively. Accordingly, any of the materials constituting the multilayer substrate has a good heat-resisting characteristic.
On the other hand, the thin-film wiring section consists of a plurality of thin insulating films made of an organic insulation material such as polyimide or the like, or a non-organic insulation material such as glass, crystallized glass, glass ceramics or the like, and various wiring patterns formed on the main surfaces of the thin insulating films and made of an active metal such as titanium (Ti), chromium (Cr), or the like and a metal having a high conductivity such as gold (Au), silver (Ag), copper (Cu), etc.
Such parts as the pads, the end portions of the I/O terminals, or the like to be connected with other elements, are provided at the exterior surfaces thereof with gold (Au) layers by, for example, plating in order to improve the wettability of the joining material (e.g., gold solder). Further, in order to prevent gold (Au) from melting down and being diffused into the joining material-at the time of joining or in order to prevent gold (Au) from changing the color due to the diffusion of gold (Au) into the base metal, the I/O terminals are provided with a nickel (Ni) layer under the gold (Au) layer. Further, in case where the conductive pad is of the kind consisting of a gold (Au) layer formed on a metalized surface made of a high melting point metal such as tungsten (w), molybdenum (Mo), or the like, a nickel (Ni) layer is provided under the gold (Au) layer for the same reason.
In the above described high density IC package, the conductive pads are generally formed from a thin film. Further, even if the pads are formed from a thick film of a high meting point metal, there are arranged adjacent thereto thin-film wiring. Those thin films are therefore inferior in heat resistance to the material constituting the above described multilayer substrate. Accordingly, in case of joining the I/O terminals such as external leads or pins, such gold solder as a gold-tin (Au--Sn) eutectic alloy, gold-germanium (Au--Ge) eutectic alloy, gold-silicon (Au--Si) eutectic alloy, or the like having a low melting point is used as a joining material.
However, by the experiments conducted by the applicants, it was revealed that any of the non-Au components contained in the gold (Au) solder such as tin (Sn), germanium (Ge), silicon (Si), or the like, reacted with nickel (Ni) of the nickel (Ni) layer at the time of joining or in use after joining, to form at the joint a brittle intermetallic compound causative of decrease of the joining strength.